In a vehicle, a transmission is interposed between an internal combustion engine and a driving wheel. This transmission changes the driving power provided to the driving wheel and the traveling speed in accordance with traveling conditions which are widely changed, so that the internal combustion engine can exhibit excellent performance. As an example of one kind of such transmission, there is a continuously variable transmission in which a transmission gear ratio (belt ratio) for transmitting power is changed by increasing and decreasing a rotational radius of a belt looped around a pulley having a fixed pulley piece fixed to a rotational shaft and a movable pulley piece movably supported on the rotational shaft so that the movable pulley piece can be moved toward and away from the fixed pulley piece to increase and decrease the width of a groove formed between the two pulleys by means of variations in hydraulic oil pressure. Such a continuously variable transmission is disclosed, for example, in Japanese Patent Early Laid-open Publication No. Sho 57-186656, Japanese Patent Early Laid-open Publication No. Sho 59-43249, Japanese Patent Early Laid-open Publication No. Sho 59-77159 and Japanese Patent Early Laid-open Publication No. Sho 61-233256.
Also, as such a continuously transmission, there is one which has a single plate type hydraulic clutch and selectively supplies power under hydraulic oil pressure control. This single plate type hydraulic clutch is controlled in various control modes in accord with a signal representing an engine speed, such as an opening degree of a throttle valve of a carburetor or the like.
In a conventional clutch pressure control device for a hydraulic clutch, there is a method for controlling clutch pressure to a target clutch pressure determined by an engine speed in a hold state of a half-clutch state, a method for controlling pressure to realize a target number of rotations determined by a throttle opening degree obtained by stepping on an accelerating pedal in a start mode, and a method for gradually increasing clutch pressure in accord with passage of time or determining clutch pressure as a minimum or maximum.
However, the hydraulic clutch is normally locked up when a vehicle is coasting at a medium or low speed where an accelerating pedal is not significantly pressed, that is, where a throttle opening degree is almost zero and a condition of traveling speed less than 5 km/hr is satisfied. Therefore, there arises an inconvenience in that, when the accelerating pedal is stepped on at that time, the engine torque generated is directly transmitted to the transmission and a vehicle body and gives vibrations or a shock to passengers.
Also, when accelerating from a reduced speed or a very low speed traveling when the hydraulic clutch is locked up, the engine speed is restrained immediately after the accelerating pedal is stepped on. As a result, an acceleration response is delayed and the state of the clutch is radically changed to generate a great shock. Consequently, the driving feeling is badly spoiled.
It is therefore an object of the present invention to realize a clutch pressure control in which, in order to eliminate the above-mentioned inconveniences, an engine braking works well while a vehicle is coasting at a medium or low speed by calculating a target clutch pressure with reference to a slip amount of a hydraulic clutch and feed-back controlling the target pressure so that the slip amount of the clutch is maintained substantially constant. When accelerating, a response to acceleration is enhanced to improve acceleration performance, and a radical change of the state of the clutch is prevented to improve the driving feeling.